In a known type of pneumatic positioner, a signal capsule is attached to one end of a vane and a relay structure is attached to the other end of the vane. A nozzle is positioned proximate to the vane intermediate the signal capsule and relay structure. The nozzle is continually pressurized by a source of compressed air. In operation, an increasing pneumatic input control signal applied to the signal capsule moves the vane toward the nozzle so as to decrease the clearance between the vane and the nozzle and thereby increase the nozzle back pressure. The back pressure, acting against a supply pressure, moves the relay structure so as to proportionately open and close air supply and exhaust valves to regulate the air supply or exhaust and provide a signal pressure differential that displaces a final control element. The amount by which the relay structure moves is determined by the distance the vane moves away from the nozzle, decreasing nozzle back pressure, and causing the motion of the relay structure to cease. The displacement motion of the final control element is fed back to the vane by a range spring. When a force balance is reached between the range spring and the input signal, the relay structure returns to a neutral position. The final control element is then in equilibrium with the input control signal. A decreasing input control signal reverses the sequence.
The nozzle essentially functions as a fulcrum in relation to the signal capsule and relay structure on the vane. The operational relationship between the signal capsule, the nozzle, and the relay structure is dependent upon the positions of these three parts along the vane. For a given position, the motions are fixed.